[Show abstract][Hide abstract] ABSTRACT: Microtubule dynamics is regulated by an array of microtubule associated proteins of which the microtubule plus-end tracking proteins (+TIPs) are prominent examples. +TIPs form dynamic interaction networks at growing microtubule ends in an EB1-dependent manner. The interaction between the C-terminal domain of EB1 and the CAP-Gly domains of the +TIP CLIP-170 depends on the last tyrosine residue of EB1. In the present study, we generated peptidic probes corresponding to the C-terminal tail of EB1 to affinity-capture binding partners from cell lysates. Using an MS-based approach, we showed that the last 15 amino-acid residues of EB1, either free or immobilized on beads, bound recombinant CAP-Gly domains of CLIP-170. We further demonstrate that this binding was prevented when the C-terminal tyrosine of EB1 was absent in the peptidic probes. Western blotting in combination with a label-free quantitative proteomic analysis revealed that the peptidic probe harboring the C-terminal tyrosine of EB1 effectively pulled-down proteins with CAP-Gly domains from endothelial cell extracts. Additional proteins known to interact directly or indirectly with EB1 and the microtubule cytoskeleton were also identified. Our peptidic probes represent valuable tools to detect changes induced in EB1-dependent +TIP networks by external cues such as growth factors and small molecules.
Journal of proteomics 04/2012; 75(12):3605-16. DOI:10.1016/j.jprot.2012.04.006 · 3.89 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: End binding protein 1 (EB1) and cytoplasmic linker protein of 170 kDa (CLIP-170) are two well-studied microtubule plus-end-tracking proteins (+TIPs) that target growing microtubule plus ends in the form of comet tails and regulate microtubule dynamics. However, the mechanism by which they regulate microtubule dynamics is not well understood. Using full-length EB1 and a minimal functional fragment of CLIP-170 (ClipCG12), we found that EB1 and CLIP-170 cooperatively regulate microtubule dynamic instability at concentrations below which neither protein is effective. By use of small-angle X-ray scattering and analytical ultracentrifugation, we found that ClipCG12 adopts a largely extended conformation with two noninteracting CAP-Gly domains and that it formed a complex in solution with EB1. Using a reconstituted steady-state mammalian microtubule system, we found that at a low concentration of 250 nM, neither EB1 nor ClipCG12 individually modulated plus-end dynamic instability. Higher concentrations (up to 2 μM) of the two proteins individually did modulate dynamic instability, perhaps by a combination of effects at the tips and along the microtubule lengths. However, when low concentrations (250 nM) of EB1 and ClipCG12 were present together, the mixture modulated dynamic instability considerably. Using a pulsing strategy with [γ(32)P]GTP, we further found that unlike EB1 or ClipCG12 alone, the EB1-ClipCG12 mixture partially depleted the microtubule ends of stably bound (32)P(i). Together, our results suggest that EB1 and ClipCG12 act cooperatively to regulate microtubule dynamics. They further indicate that stabilization of microtubule plus ends by the EB1-ClipCG12 mixture may involve modification of an aspect of the stabilizing cap.
[Show abstract][Hide abstract] ABSTRACT: Like many asymmetrically dividing cells, budding yeast segregates mitotic spindle poles nonrandomly between mother and daughter cells. During metaphase, the spindle positioning protein Kar9 accumulates asymmetrically, localizing specifically to astral microtubules emanating from the old spindle pole body (SPB) and driving its segregation to the bud. Here, we show that the SPB component Nud1/centriolin acts through the mitotic exit network (MEN) to specify asymmetric SPB inheritance. In the absence of MEN signaling, Kar9 asymmetry is unstable and its preference for the old SPB is disrupted. Consistent with this, phosphorylation of Kar9 by the MEN kinases Dbf2 and Dbf20 is not required to break Kar9 symmetry but is instead required to maintain stable association of Kar9 with the old SPB throughout metaphase. We propose that MEN signaling links Kar9 regulation to SPB identity through biasing and stabilizing the age-insensitive, cyclin-B-dependent mechanism of symmetry breaking.
[Show abstract][Hide abstract] ABSTRACT: The ends of growing microtubules (MTs) accumulate a set of diverse factors known as MT plus end-tracking proteins (+TIPs), which control microtubule dynamics and organization. In this paper, we identify SLAIN2 as a key component of +TIP interaction networks. We showed that the C-terminal part of SLAIN2 bound to end-binding proteins (EBs), cytoplasmic linker proteins (CLIPs), and CLIP-associated proteins and characterized in detail the interaction of SLAIN2 with EB1 and CLIP-170. Furthermore, we found that the N-terminal part of SLAIN2 interacted with ch-TOG, the mammalian homologue of the MT polymerase XMAP215. Through its multiple interactions, SLAIN2 enhanced ch-TOG accumulation at MT plus ends and, as a consequence, strongly stimulated processive MT polymerization in interphase cells. Depletion or disruption of the SLAIN2-ch-TOG complex led to disorganization of the radial MT array. During mitosis, SLAIN2 became highly phosphorylated, and its interaction with EBs and ch-TOG was inhibited. Our study provides new insights into the molecular mechanisms underlying cell cycle-specific regulation of MT polymerization and the organization of the MT network.
The Journal of Cell Biology 06/2011; 193(6):1083-99. DOI:10.1083/jcb.201012179 · 9.83 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Alternative splicing plays an important role in generating proteome diversity. The polypyrimidine tract-binding protein (PTB) is a key alternative splicing factor involved in exon repression. It has been proposed that PTB acts by looping out exons flanked by pyrimidine tracts. We present fluorescence, NMR, and in vivo splicing data in support of a role of PTB in inducing RNA loops. We show that the RNA recognition motifs (RRMs) 3 and 4 of PTB can bind two distant pyrimidine tracts and bring their 5' and 3' ends in close proximity, thus looping the RNA. Efficient looping requires an intervening sequence of 15 nucleotides or longer between the pyrimidine tracts. RRM3 and RRM4 bind the 5' and the 3' pyrimidine tracts, respectively, in a specific directionality and work synergistically for efficient splicing repression in vivo.
Proceedings of the National Academy of Sciences 02/2010; 107(9):4105-10. DOI:10.1073/pnas.0907072107 · 9.67 Impact Factor